The invention describes an air-breathing fuel cell for the oxidation of ions with air or oxygen, having an anode half cell and a cathode half cell. A first ion-conducting membrane and a second ion-conducting membrane is introduced between the half cells, and the second ion-conducting membrane is coated at least in regions on the side orientated towards the cathode half cell with a catalyst for the reduction of oxygen. According to the invention, the air-breathing fuel cell is characterised in that an oxidation zone for the oxidation of ions with negative standard electrode potential is provided between the ion-conducting membranes.
A typical example of an air-breathing fuel cell from the state of the art is the vanadium/air fuel cell (DE 692 17 725 T2), termed here redox battery. In the case of this special embodiment, bivalent vanadium is oxidised to form trivalent vanadium at the anode, oxygen being reduced at the cathode and reacting with protons to form water.
The chemical reactions are the following:Anode: V2+→V3++e− E0=−0.255 VCathode: O2+4H++4e−→2H2O  E0=+1.2 Vcathode is effected with the aid of a catalyst on a carbon electrode. The catalyst/carbon mixture is applied on the cathode side of the membrane (membrane electrode unit) and is in contact with a gas diffusion layer which consists of carbon and is in contact in turn with a carbon plate. The anode half cell consists of a carbon plate which is in contact with a porous carbon material. The porous carbon material serves for enlarging the surface and hence for increasing the power density. The porous carbon material, typically a graphite felt, is in contact with the membrane which has no catalyst coating on the anode side.
Furthermore, in this example from the state of the art, an acidic solution of bivalent vanadium ions is pumped through the anode half cell, whilst air is conducted through the cathode half cell. A terminal voltage is set between the carbon electrodes of the two half cells. If the circuit is completed, electrons flow from the anode via the consumer to the cathode.
The ion-conducting membrane is not 100% impermeable relative to the media so that the acidic solution of bivalent vanadium ions passes through the membrane to the applied catalyst layer. Because of the potential differences of the partial reactions, the following reaction thereby takes place on noble metal particles:V2+V3++e− E0=−0.255 V2H++2e−H2  E0=±0.0 V2V2++2H+→2V3++H2↑
Because of the production of gaseous hydrogen in the catalyst layer, a change in the latter is effected by for example detachment of individual particles, which leads to the speed of the oxygen reduction reaction and hence the total power of the cell reducing rapidly. In addition, this effect is accelerated by current conduction through the cell and with accompanying electromigration of vanadium ions to the cathode.